نوع مقاله : مقاله پژوهشی

نویسندگان

1 آب و هواشناسی دیرینه، دانشکده جغرافیا، دانشگاه تهران و عضو انجمن کواترنری ایران، ایران

2 آب و هواشناسی، دانشکده جغرافیا، دانشگاه تهران، ایران

3 خاک شناسی، دانشگاه علوم کشاورزی و منابع طبیعی، گرگان، ایران

4 مهندسی منابع طبیعی، مدیریت مخاطرات و زیست محیطی، سازمان زمین شناسی و اکتشافات معدنی کشور، ایران

5 خاک شناسی، دانشگاه کلن، کلن، آلمان

6 آزمایشگاه محیط و دیرین مغناطیس، سازمان زمین شناسى و اکتشافات معدنى کشور، تهران، ایران

چکیده

شناخت علل تغییرات اقلیمی جهانی یکی از چالش‌های مهم علمی است. بازسازی آب‌وهوای گذشته از بایگانی‌های مختلف رسوب و یخ نشان می‌دهد که آب‌وهوا می‌تواند به طور چشمگیری در بازه‌های زمانی متفاوت، از چندین دهه تا هزاره، تغییر کند. گردوغبار بادی، که با تغییرات آب‌وهوایی جهانی در ارتباط است، یکی از مؤلفه‌های مهم سیستم آب‌وهوایی است که تغییرات ناگهانی گذشته را نشان می‌دهد. لس‌ها به طور بالقوه می‌توانند تغییرات دیرینه محیطی را طی چندین چرخه یخبندان ثبت کنند. در این پژوهش، برش رسوبی سعدآباد، واقع در استان گرگان، جهت بررسی آب‌وهوای دیرینه، مربوط به MIS ۵e (۱۴۷۰۰۰ تا ۵۷۰۰۰ سال پیش) شمال‏شرق ایران، با توجه به نبود اطلاعات کافی در خصوص آب‌وهوای دیرینه و سیستم‌های تأثیرگذار بر آن در این دوره زمانی، مطالعه شد. بدین منظور ابتدا مطالعات کتابخانه‏ای و میدانی و سپس مطالعات آزمایشگاهی صورت گرفت و تعداد ۹۲ نمونه به فواصل ۱۰ سانتی‌متری از توالی مورد نظر تهیه و برای همه آن‌ها پذیرفتاری مغناطیسی اندازه‏گیری شد. بر اساس نتایج به‌دست‏آمده، نمونه‏ ها دارای نوسانات افزایشی در بخش‏های پالئوسولی و کاهشی در بخش‏های لسی بودند. نتایج حاکی از آن بود که برش رسوبی سعدآباد در طول حدود ۱۴۷۰۰۰ تا ۵۷۰۰۰ سال گذشته حدود سه دوره آب‌وهوایی گرم و مرطوب و سه دوره سرد و خشک داشته است. در دوره آب‌وهوایی گرم و مرطوب لایه‏های خاک دیرینه و شبه‌خاک دیرینه با میزان پذیرفتاری مغناطیسی بالا و در دوره آب‌وهوایی سرد و خشک لایه‏های لس با میزان پذیرفتاری پایین تشکیل شده است. آب‌وهوای دیرینه و پی بردن به جزئیات تغییرات آن در گذشته به برنامه‌ریزی‌های مدیریتی و زیرساختی و ایجاد سناریوهای منطقی اقلیمی در کشور کمک شایانی خواهد کرد.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

Reconstruction of the Late-Quaternary climate change using magnetic susceptibility of Saadabad-Gorgan loess deposits

نویسندگان [English]

  • Sahar Maleki 1
  • Ghasem Azizi 2
  • Farhad Kharmali 3
  • Reza Shahbazi 4
  • Martin Kohl 5
  • Atke Aslani 6

1 Paleoclimatology, Faculty of Geography, University of Tehran and member of Quaternary Society of Iran, Iran

2 Hydrology and Meteorology, Faculty of Geography, University of Tehran, Iran

3 Soil Science, University of Agricultural Sciences and Natural Resources, Gorgan, Iran

4 Natural resources engineering, risk and environmental management,, Geological Survey of Iran

5 Geology, University of Cologne, Cologne, Germany

6 Environmental and Paleomagnetism Laboratory,, Geological Survey of Iran

چکیده [English]

Introduction
 
One of the processes of the Quaternary era is the glacial and interglacial periods. In the glacial period, loesses deposited and in the interglacial period, Paleosoils were formed. The northern part of Iran is geographically the same as countries such as China, a large part of which has been covered with loess sediments during the Pleistocene glacial period. Loess sediments in northern Iran reflect several cycles of climate changes and the evolution of the earth's appearance for the middle to late Quaternary period. In this region, the Loes-Paleosoil sequences are one of the most important terrestrial archives of climate change and provide a bridge between Southeast European and Central Asian loess sediments. To reconstruct paleoclimate changes, magnetic susceptibility is used as a climate proxy. The high magnitude of the magnetic susceptibility indicates the large volume of magnetic minerals. Chemical weathering causes the formation of magnetic minerals such as magnetite and maghemitite. These minerals are abundant in Paleosoils and can be determined by analyzing the magnetic susceptibility of Loess and Paleosoils. The magnetic properties of the Loess-Paleosoil sequence are considered to be evidence of paleo precipitation and weathering.
Study area
Geographical location of Saadabad sedimentary section in the northern region is 36° 49' N and 54° 22' E, at an altitude of 140 m a.s.l. Saadabad sedimentary section is located in the construction zone of Alborz and in terms of the main sedimentary-structural zones of Iran is part of the southern Caspian coast. This zone includes areas, which are located in the north of Alborz fault and block the Caspian Sea coast on the coast of Iran and are covered to the east with thick layers of loess.
Methods
In the field work that was carried out in May 2017, after determining the exact location of the sedimentary section, sampling was performed. Before sampling, aerated sedimentary layers are dug up and removed to reveal fresh deposits. Then, using the meter, the layers were divided into 10 cm intervals. 92 samples were prepared at 10 cm intervals from the sequence. Measurement of natural residue magnetic (NRM), by the model rotating magnetometer (JR-6A), and measurement of magnetic susceptibility by magnetic intensity measuring device, in the magnetometry laboratory of the Geological Survey of Iran.
Results
Table 1 shows the measured values of the magnetic parameters. Due to the length of the table, only a few examples are given in the table.
Since high magnetic susceptibility can indicate a greater concentration of magnetic minerals in the sample, it can be concluded that the paleosils of the sequence studied have more magnetic materials than the loesses, which are inside the soils. They can be the result of podogenesis processes. Because rising temperatures and humidity cause soil erosion processes and increase oxidation and thus increase the amount of magnetic materials, it can be concluded that in paleosoils with higher magnetic susceptibility, warm and humid climates dominate in the environment. Micromorphological studies of directional clay layers, Fe-Mn hydroxide, show that the palaeosoil of section 5 is moderate to well developed. As the climate changes to higher temperatures and higher humidity, weathering and pedogenesis also increase, leading to the development of a pedogenic oxidation environment as a result this produce the formation of tiny magnetic grains such as maghemite and magnetite in this oxidizer environment and thus increases the magnetic susceptibility. In Saadabad, the high amount of xlf along with the high percentage of xfd suggests that many of the ultra-fine maghemite and magentite grains may have been formed during pedogenesis under a long, humid, warm climate. The findings show that the different behaviors of magnetic susceptibility between the loess of the drier and wetter areas are mainly caused by their different pathogenic environments, which in turn are related to local topography and climatic conditions.
Conclusions
High levels of xlf and xfd indicate high precipitation during the formation of Paleosoils. Post-sedimentation processes may increase the amount of MS by producing new ferromagnetic minerals during the oxidation of wheathered soils, and may also reduce the amount of MS by reducing the processes. The clay material in which the soil is made is the main factor influencing the change in magnetic mineralogy and magnetic susceptibility. This study also shows that magnetic susceptibility is a complex parameter and its use as a precipitation control has certain limitations and conditions, and when the magnetic properties of the loesses are used for paleo climate reconstruction, more attention should be paid to topography, environment. Sediment and weather factors.

کلیدواژه‌ها [English]

  • Magnetic Susceptibility
  • Loess
  • Paleosoil
  • paleoclimate